Field of the Invention
The present invention relates to an image forming apparatus including a charging unit of a proximity charging system to which a charging voltage having an AC voltage superimposed on a DC voltage is applied.
Description of the Related Art
In recent years, for a charging system for an image forming apparatus, a proximity charging system has become mainstream. In the proximity charging system, for example, a roller type charging unit is disposed in proximity to a photoconductor drum so as to contact or not to contact a surface of the photoconductor drum. A charging voltage having an AC voltage superimposed on a DC voltage is applied to the charging unit so that the surface of the photoconductor drum can be uniformly charged.
In the above-described image forming apparatus, a peak-to-peak voltage value Vpp of an AC voltage Vac to be superimposed on the charging voltage is derived by charging voltage determination control. Conventionally, this type of charging voltage determination control is described in, for example, JP 2008-107605 A. In JP 2008-107605 A, a peak-to-peak voltage value Vpp of an AC voltage Vac is derived by gradually increasing the peak-to-peak voltage value Vpp of the AC voltage Vac from an initial value until a current value detected by a current detecting unit is saturated. If the AC voltage Vac exceeds clamping voltage during this control, the peak-to-peak voltage value Vpp is set to an initial voltage. On the other hand, if a temperature and a humidity exceed their predetermined reference values, the peak-to-peak voltage value Vpp is set to a value smaller than a peak-to-peak voltage value Vpp that is obtained when one of the temperature and humidity is less than its reference value.
In JP 2008-107605 A, it is premised to use a photoconductor drum having a hard photoconductor film, e.g., an a-Si (amorphous silicon) photoconductor. Therefore, JP 2008-107605 A does not take into account changes in detected current value caused by wastage of the film thickness of the photoconductor drum.
If the photoconductor film is hard, it is disadvantageous to remove discharge products, etc., adhered to the surface of the photoconductor drum. Thus, it is desirable to use a photoconductor drum whose film thickness is wasted moderately during cleaning, etc. However, if the charging voltage determination control described in JP 2008-107605A is adopted when such a photoconductor drum is used, the current detecting unit may detect an inaccurate current value due to wastage of the film thickness, which may result in a control unit setting an inaccurate peak-to-peak voltage value Vpp.